Grounded plate amplifier



Aug. 1941- J. w. CCNKLIN EI'AL 2,252,458

GROUNDED PLATE AMPLIFIER Filed Nov. 15, 1938 7t r I m/ ar Patented Aug. 12, 1941 UNITED STATES PATENT OFFICE" GROUNDED PLATE AMPLIFIER James W. Conklin, Audubon, and Samuel Gubin, Erlton, N. J assignors to Radio Corporation of America, a corporation of Delaware Application November 15, 1938, Serial No. 240,444

4 Claims. ((31.179-171) This invention relates to improvements in grounded plate amplifiers and more particularly to a grounded plate amplifier suitabl for ultra high frequency amplification.

Among the objects of this invention are the provision of a grounded plate amplifier of the push pull type which is particularly suitable for use at ultra high frequencies; the provision of means for effectively applying input potentials to the grid of a vacuum tube in series with the output potentials; the provision of an effectively shielded ultra high frequency amplifier; the provision of an amplifier which is free from undesirable parasitic oscillations at ultra high frequencies; and the provision of means for neutralizing a push pull amplifier in such a manner that the neutralizing condensers are not subjected to ahigh voltage.

since the grounded anode electrode itself constitutes a shield around the cathode electrode. When a tube is operated in this manner th input voltage can not be applied to the grid in the usual manner, for the reason that the cathode electrode is not at a constant potential but is varying over a voltage range which is greater than the input voltage by the amplification of the tube.

In accordance with this invention, it is proposed to apply to the grid electrodes of a pair of amplifier tubes an input voltage which is effectively in series with the output potential which appears between between the cathode electrodes and ground.

This invention will be better understood from the following description when considered in connection with the accompanying drawing. Similar reference numerals refer to similar parts throughout the several figures.

Figure 1 is an electrically equivalent circuit of one embodimentof my invention; Figure 2 is a view, partly in section, of apreferred embodiment of my invention; and Figure 3 is a view, partly in. section, of an alternative embodiment of my invention.

Referring to Fig. 1, a pair of amplifier tubes 5 and l have their anode electrodes 9 and II bypassed to ground by a capacitor l3. Anode potential is supplied by a battery I5 or the like. An inductor which comprises two sections, l1 and i9, is connected between the cathode electrodes of the tubes. The midpoint 2! of the inductor is grounded. Closely coupled to inductors I! and H] are, respectively, two inductors 23 and 25, the outer terminals of which are connected respectively to the grids of the two tubes. The inner terminals of the inductors 23 .and 25 are connected to a source of input voltage such as the secondary of an input transformer 21. A grid bias battery 43 is connected between the midpoint of the secondary winding and ground. The input voltage is thus applied in phase opposition to the two grids, and the output voltage, which is developed across the inductors l1 and I9, is induced likewise on the two grids. The potential difference between the grid and cathode electrodes of each tube is, therefore, dependent entirely upon the input voltage. Amplification is achieved since a small input voltage is effectively impressed between grid and cathode and a large output voltage is available across inductors l1 and I9.

Referring, now, to Fig. 2, we have shown a practical application of our invention. A cylindrical metal shield 29 encloses two hollow conductors 3| and 33 which are of substantially less diameter than the shield. The hollow conductors are parallel to the axis of, the shield. One end of the shielding cylinder 29 is closed by an end plate 35 on which are mounted the hollow conductors 3| and 33. The two hollow conductors constitute a resonant tank circuit for theamplifier. The tank circuit is tuned by a movable shorting bar 31, or, alternatively, by a variable capacitor connected between the two conductors.

Two amplifier tubes 5 and l are mounted adjacent the free ends of the hollow conductors. The cathode electrodes of the tubes are connected directly to respective ends of the hollow conductors. The anode electrodes. are connected together and bypassed to the shield 29 by capacitors 39. Positive anode potential is supplied by a battery [5 or the like. The grids of the two tubes 5 and I are connected to the terminals of an input circuit, such as a transformer 21 or the like, by conductors which pass within respective hollow conductors 3| and 33. Grid bias is obtained from a battery 43, or the like, which is connected between the shield 29 and the midpoint of the secondary of. the input transformer 21. Since, as noted above, the grid and cathode electrodes are at a high radio frequency potential, it is also necessary to bring the filament leads 45 through the hollow conductors.

The similarity between the embodiment illustrated in Fig. 2 and the schematic diagram of Fig. 1 is seen when it is understood that the parallel hollow conductors 3| and 33 correspond, respectively, to inductors I! and I9 of Fig, 1, and that the conductors which connect the grids to the input transformer, being within the hollow conductors, are equivalent to the inductors 23 and 25 of Fig. 1.

In order to prevent self-oscillation, it is necessary to provide neutralization. We have accomplished this in the present invention by connecting capacitors 4'! and 49 respectively between the cathode of one tube and the grid of the other tube by means of conductors which extend respectively through the hollow conductors 3| and 33, and connect at the secondary of the input transformer to the lead from the grid of the other tube. The neutralizing conductors within the hollow tank circuit conductors 3| and 33 are equivalent to the inductors shown at 5| and 53 in Fig. 1. It is to be noted that there is no direct current potential across the neutralizing condensers, and further, that the alternating current potential is only that due to the input voltage. Thus the need for capacitors of high voltage rating for neutralization is avoided in the present invention.

The tank circuit which includes inductors 3i and 33 is tuned by the interelectrode capacities of the tubes, so that the actual length of the conductors is somewhat less than a quarter wave length. A network balancingcapacitor 55 is connected. between the two neutralizing condensers 41 and 49 to make the electrical length of the neutralizing lead, which is within the hollow conductors, equal to the electrical length of the hollow conductors.

In tubes which are characterized by the use of a large external anode electrode, the anode-grid capacity is frequently greater than the cathodegrid capacity. Since, in the present invention, the cathode-grid capacity is that which must be neutralized, it is possible to employ neutralizing capacitors of less capacity than has previously been possible. Due to the low voltage across the neutralizing capacitors, and due to their low capacity, the stored energy of the circuit is small. This tends to increase the efiiciency of the amplifier at high frequencies and makes it more suitable for the amplification of signals which are modulated over a wide band. Parasitic oscillations are reduced because neutralization is equally effective at all frequencies.

A further advantage of the present system is realized by reason of the fact that the capacitive load on the tank circuit is low since the tank circuit is connected to the cathode and grid electrodes instead of to the anode electrodes which have a high capacity to ground. This is desirable for the reason that the Q of the resonant circuit is thereby increased. In addition, the use of grounded anodes makes possible an extremely short ground connection which is diflicult to obtain in grounded cathode systems due to the length of the cathode lead within the tube itself.

A modification of our invention is shown in Fig. 3. If it is desired to use an amplifier tube having a filament which also serves as a cathode, we have provided a convenient means for supplying the energizing potential to the filaments by providing four hollow conductors 51, 59, BI and 63 which are arranged in pairs, each conductor of a pair being insulated from but closely coupled to the other conductor of that pair. While the drawing shows the conductors substantially spaced so as to permit the connections to be shown more clearly, in practice they should be closely coupled. This is indicated in the figure by the dotted capacitors connected between adjacent conductors. The four conductors are bypassed to the shield at one end so that the two conductors of each pair may be utilized to carry the filament current to the tubes. At radio frequencies, the two conductors of each pair are effectively one and constitute a resonant circuit in the manner described with respect to Fig. 2. The system is otherwise similar to Fig. 2 and it is not necessary to explain it further.

Other modifications may be made Within the scope of this invention. For example, the shielding member 29 is not inherently necessary to this invention. Likewise, the tank circuit inductors may take the conventional form of lumped inductance somewhat in the manner illustrated in Fig. 1.

We claim as our invention:

1. In an amplifier, a pair of thermionic tubes having anode, cathode and grid electrodes; a half wave resonant line connected between said cathode electrodes and having its midpoint grounded; means for grounding said anode electrodes for alternating current voltages; means for applying alternating current voltages in phase opposition between said grid electrodes and ground; said means including a conductor passing within but insulated from said resonant line; a neutralizing capacitor connected from the grid of each tube to the cathode of the other, said connection likewise passing through said resonant line; and means for obtaining an am plified alternating current voltage from said resonant circuit.

2. An amplifier as described in claim 1 which is further characterized by connections passing within said resonant line for energizing said cathode electrodes.

3. In an amplifier, a cylindrical shield enclosing two hollow conductors of substantially less diameter than said shield, said hollow conductors being parallel to the axis of said shield and connected to said shield and to each other at one end; a pair of thermionic tubes having grid, cathode and anode electrodes; means connecting said cathode electrodes respectively to the free ends of said hollow conductors; means for grounding said anode electrodes with respect to alternating current voltages; a source of input voltages a conductor passing through one of said hollow conductors connecting each of said grid electrodes to said source of input voltage, neutralizing capacitors respectively connected between the grid of each tube and the cathode of the other by conductors which pass through said hollow conductors, said hollow conductors and the electrode capacities of said tubes comprising a circuit resonant at the frequency of said input voltage; and means for obtaining an output voltage.

4. In an amplifier, a cylindrical shield enclosing four hollow conductors of substantially less diameter thansaid shield, said hollow conductors being parallel to the axis of said shield and arranged in pairs, each conductor of a pair being insulated from but closely coupled to the other conductor of that pair, each conductor being closely coupled to said shield at one end; a pair of thermionic tubes having anode, cathode and grid electrodes; means respectively connecting said cathode electrodes between the free ends of said pairs of hollow conductors; means for grounding said anode electrodes With respect to alternating current voltages; a source of input voltages, a conductor passing through one of each pair of hollow conductors connecting respective grid electrodes to said source of input voltage; 10 

